[Maarten88]

Any competitive sailor or foil-racer knows that the underwater surface has the least friction and best laminar flow when sanded with fine-grid sandpaper, around 1000 to 1500 grid.

It always surprised me that this was not true in air and airplane wings were supposedly best when glossy. So now it turns out that this is indeed not true, and airfoils also benefit from micro-roughness for lowest friction.

Now the surprising question to me is how is it possible that something so simple was not known in this very well-researched and well-funded field. It probably was known, just not by the paper-publishing researchers.

[otterdude]

The core tenant of the paper is that roughness reduces drag IN the transition zone. A very small region of the total flow.

Thats the region between laminar and turbulent flow. Laminar flow is typically 5x less drag than turbulent, and will be encountered about a Reynolds number of 500K-1M (ratio of inertial flow to viscous flow).

Surfboards will have a Reynolds number of 10^7 which is entirely turbulent.

A Cessna aircraft will have a Reynolds number of 1-5x10^6.

[mike_hock]

> core tenant

And Lady Mondegreen.

[bonsai_spool]

Tenet is the word you mean

[dsr_]

And grit, not grid.

[nnevatie]

swoosh swoosh - that's the sound of things traversing backward in time.

[taylorius]

I think you mean hsoows hsoows

[Maarten88]

> Surfboards will have a Reynolds number of 10^7 which is entirely turbulent.

A fin, foil or daggerboard below the board/boat is operating well within the range of Reynolds' numbers where laminar flow is relevant.

[otterdude]

The vertical stabilizing surface of these elements is really insignificant to the entire surface of a board. Combining drag coefficients is done with the wetted surface area.

In truth there's some contamination from the upstream flow. Stabilizing elements are behind the center of pressure, so they will see the most "diry flow"

[Maarten88]

You are thinking about a slow boat in displacement mode, or a wave-surf board with very small fins. But I can tell you that the wetted surface area of my wing-foil board is exactly zero after takeoff, and all wet surfaces of the foil are, for a significant percentage, in laminar flow. Same for a windsurf board planing: just the last 50cm of the board is touching the water and the fin is extremely significant for drag.

[lymbo]

I have 2 foil drives for wake thieving and a SUP for downwinding and I’m just excited that someone on HN is talking about foiling. Such an underrated sport.

[Arodex]

Water is fairly viscous, and when you try to pull through too fast you completely change regime due to cavitation.

In comparison, from my days studying aerodynamics for RC soaring, air has a wider range of "viscosities" (represented by the Reynolds number) depending on the scale of your aeroplane and the speeds you intend to go through the atmosphere. The aerodynamic ideal or what count as useful tricks (winglets, dimples) can be fairly different for a a golf ball compared to a RC airplane compared to a commercial jet compared to a fighter jet...

[naasking]

Water is also largely incompressible. The fluid dynamics are just too dissimilar to air to carry over simplistic assumptions.

[warumdarum]

Wasnt there something about building abblative vortexes that convert the friction into rotation and are then discarded at the edge of the surface?

[stasomatic]

Asking as a complete neophyte - how does this reconcile with modern war planes being inherently unstable as far they flight dynamics go, without their enormous thrust capabilities? I’m just curious, I know nothing about the subject, but it seems that the solution we came up with is thrust, baby.

[xyzzy_plugh]

A key difference is that war planes occasionally want to be able to rapidly change their trajectories.

With sufficient thrust you can fly around in a cube.

[stasomatic]

Would you a have a link that would show case something like that? It feels like only a T-1000 would be able to make any rapid coherent decisions under such load. Thank you.

[murderfs]

Instability just means that they don't naturally return to stable flight. Fighter jets benefit from this because when you want to make a maneuver, you're not fighting the plane's natural inclination to stay where it's pointing. You don't need particularly powerful hardware to do this kind of control, quadcopters are an even more obviously inherently unstable example, because any thrust imbalance will immediately make it roll. Quadcopter control loops only need to run at a few hundred hertz to achieve stability.

[soco]

Enter drone piloting, enter autonomous AI... We are maybe approaching that stage, not necessarily with shiny exoskeletons, but not-human nevertheless.

[stasomatic]

They can’t “want “. The pilot may want, but can she, under these loads?

[simne]

That's simple - to make quick rotation, you must apply force (torque) and beat inertia - from mathematics could know, the farther from center of mass the more energy will spend to rotate at same speed.

So, to rotate faster, you need larger control surfaces.

From other side, traditionally, self-stabilize spent at least 1% of energy (on small planes normal up to 10%). What all this mean - with 10 000kg of total weight, your control surface will constantly make 1000kg of force to just fly, but when need to turn, will need significantly more than 1000kg, that's all.

Old planes need self-stabilize, because constant corrections was very time consuming, but modern have powerful computers and could provide artificial stabilization - current 1kg computer could provide same stabilization as control surface constantly making 1000kg of force.

[bombcar]

Unstable fighters gives them much more maneuverability at the cost of “not returning to straight and level flight” that normal planes have.

It’s not directly related to how the wind goes over the wings.

[stasomatic]

I am curious about humans operating machinery outside of our inherent performance envelopes. Do we have enough runway beyond switching arrow tips from flint to SS?

[rayiner]

> It always surprised me that this was not true in air and airplane wings were supposedly best when glossy.

I was an AE major and I don’t recall ever learning that airplane wings were best when perfectly smooth, even as a simplification in undergraduate courses. We were taught that drag is reduced by maintaining an attached laminar flow.

Airplane wings are glossy because they’re metal (or CFRP) and painted for durability and corrosion and UV resistance.

[zobzu]

as usual these things are presented as new and revolutionary but aren't actually.

the specific process and implemention however are usually newer or slightly different from before.

this is our sensationalistic based society - any iterative progress, or sometimes even copy, is explained as a revolution.

now show me a 737 using 40% less fuel - guess what - that wont happen - however, perhaps we'll get a slightly better process to create aircraft skins. keep in mind you cant re-sand a fuselage every week, it needs to work reliably with no maintenance.

[phonon]

"... this flying wing will burn 50% less fuel than today's jets..."[1]

[1]https://time.com/7292452/jetzero-low-carbon-air-travel/

[colordrops]

Yeah I'm pretty sure I remember reading something in a pop science magazine 20 or 30 years ago when MEMS nano structures were all the rage and how they were gonna use mass arrays of them on airplane wings to somehow increase flow

[greggsy]

Not uncommon to hear bold claims with every new and emerging technology that isn’t well understood by the media or general public. The excitement over nanobots seems to have run its course (for now?). Blockchain managed to find its way into every market imaginable. Battery technologies have consistently delivered bold claims on an almost yearly cycle, but we have at least seen incremental improvements. AI is obviously the worst offender in the current timeline.

[aeternum]

I wonder how quickly airlines will adopt sanded/rough wings. It's also interesting that the efficiency of winglets were known for quite awhile but only somewhat recently have nearly all airliners adopted them.

[rogerrogerr]

It’s probably operationally easier to keep surfaces smooth than to keep them a specific amount of roughness.

[greggsy]

It’s presumably easier to keep a smooth surface clear of bugs, dust and ice too.

[swasheck]

yeah. what are the effects of too much roughness? may be safer and easier to maintain at smooth than at a specific roughness spec

[delecti]

Also matters a bit what happens to a surface that they don't do anything to. Does a precisely rough surface get too rough or too smooth? Does a precisely smooth surface get rougher in a way that's beneficial?

Could be the case that in-practice this means they just worry less as their perfectly smooth planes get a bit rough.

[PaulRobinson]

At least a decade.

I remember people could smoke on planes. On some airlines seat backs and bathrooms had cigarette ashtrays in them. Smoking was phased out between 1988 and 2000, with most airlines being smoke free in the mid-1990s.

But the ashtrays persisted well into the 2000s. Two reasons: they needed to refresh the cabins, which is on a longer maintenance cycle done every few years, and before that, they needed replacement seats and bathroom fittings without the ashtrays. That meant tests, regulatory approval, all sorts.

For ashtrays being removed.

Winglets are a similar story. They're an addition, but they needed test flying and type approval before they could be added to the maintenance cycle rotation and get added to aircraft.

This is a bigger change. Boeing and Airbus (and others), are going to need to design it, push it through CFD, build different variants, test fly them, get them through regulatory approval and then... well, existing aircraft are probably not going to get these. Too expensive, too hard.

What's going to make more sense is a new aircraft - even if it's a variant type like the 737-MAX or the A320-Neo or whatever - where they approve the type modification as a whole, but it's not a retrofit to an existing airframe, will help manufactures sell more aircraft, airlines don't need to ground existing fleet and over time the fuel efficiencies get involved.

[bitdivision]

The FAA still requires ashtrays in bathrooms interestingly. To avoid those who do smoke there using the trash and causing a fire:

  Regardless of whether smoking is allowed in any other part of the airplane, lavatories must have self-contained, removable ashtrays located conspicuously on or near the entry side of each lavatory door, except that one ashtray may serve more than one lavatory door if the ashtray can be seen readily from the cabin side of each lavatory served

https://www.law.cornell.edu/cfr/text/14/25.853

[thaumasiotes]

> For ashtrays being removed.

I don't think it's safe to generalize from this to a functional aspect of the plane. Removing the ashtrays serves no purpose, so there's no cost to letting it wait for a decade or two. Improving the aerodynamics does serve a purpose and might be done faster.

[bombcar]

At least one total loss was caused by a waste fire in the lavatory from a cigarette-the ashtrays are mandatory safety equipment and the FAA won’t let you fly without them operable.

[stackghost]

Modifications to an approved type design, especially for commercial passenger aircraft, are an intensely bureaucratic and thus very expensive process. This is part of the reason why product cycles are long.

[larusso]

I thought that shark skin foil was a thing for years. Where they tried to emulate the micro roughness of shark skin.

[plorg]

The article says the investigators identify this as something fundamentally different than the shark skin effect.

[lonely_wanderer]

The article contrasts it with a manufacturing process known as “shark skin.” I wonder if it’s entirely identical to the skin of a shark or a marketing term?

[larusso]

Sorry the article is paywalled and I reacted to the comment about sanded surfaces. I remember seeing documentation or other nature documentation’s about the shark skin effect. I had the chance to touch a shark and a ray in an aquarium but never felt the shark skin foil. I assume the properties would be the same though.

[dilawar]

> and airfoils also benefit from micro-roughness for lowest friction.

I thought this was known to some extent that smooth surfaces are not always the best e.g. golf balls have dimples on them? No?

[dilawar]

Never mind. I didn't read the article (paywalled) and someone in the comments below answered this exact point.

[bookofjoe]

https://archive.ph/YwAky

[zobzu]

dimples are used for stability and lift, not for friction reduction / low cx

[biinjo]

So you’re telling me I should sand my carbon fiber wing foil mast and wing with 1000-1500 grid sand paper for better gliding results?

I’m fairly new to the sport and never heard of this yet.

[freetinker]

We’ve known about this for a while I feel from dimples on a golf ball.